Which lead screw spec should I choose?

[analogman]

I am shopping for a Z lead screw for my CNC router project. The design calls for a 4 start 1/4" lead. Unfortunately the supplier only has 2 start 1/4" lead or 4 start 1/2" lead. Which should I go for?

thanks

[ger21]

The 2 start 1/4" lead would be equivalent, but slightly less efficient.

[analogman]

Hey thanks for the input. Had some info from the designer of the CNC as well. He said:

You can use either, as long as you understand a few points.

The 1/4" lead means it takes 4 motor turns to move 1". The reason a 1/4" lead was called out in the beginning was that there was a goal for finer resolution in the Z axis, and also, that the motor sized called out would not be able to handle the load if one used a 1/2" lead or 2 turns for 1" of travel.

The bottom line is that now we have available at an affordable cost, larger motors. So, "pulling" the 1/2" lead screw is completely possible and will in effect offer the same exact resolution as X and Y.

So, if you already have picked motors and ended up with something in that 150oz to 200oz range, it is likely that you should stick with the 1/4" lead. If you found a good deal on a 400oz motor for the z axis, it would have no trouble with the 1/2" lead.

Regarding "starts"...... The idea with having additional, separate "starts" was that it generally made the "slide" action of the nut easier because their is an increased angle in the thread. It also is supposed to let a screw and nut last longer by having more surface area in contact at all times. Either is fine. It would take many, many years to literally wear a screw or nut out, and the nuts because they are acetal will always wear first.

So, 1/4" lead for the smaller motor...... or 1/2" lead if you can end up using a larger motor (in Z).